Traffic alerting systems (e.g. Traffic Information Systems (TIS), Traffic Advisory Systems (TAS), Traffic Collision Avoidance Systems (TCAS), and Automatic Dependent Surveillance Broadcast (ADS-B) systems) are implemented in aircraft to monitor the location, speed, and heading of near-by aircraft and to alert a pilot to any aircraft that may present a threat of collision or other hazard. These systems all have a similar problem: the sensitivity needed enroute is different than that needed in the terminal environment. FIG. 1 shows an aircraft 102 and the size (i.e., sensitivity) of the area covered by traffic alerting systems. The terminal sensitivity area 104 is smaller than the enroute sensitivity area 106. Enroute, traffic alerting systems need to detect and alert conflicts at longer ranges due to the faster closing speeds encountered. There is also a lower density of traffic intruders in the enroute environment. This differs greatly from the terminal or airport environment where there is a higher density of traffic, which is slower moving. If the sensitivity that is optimal for the enroute environment is used in the terminal environment, there will be an increased number of false alarms, where a traffic intruder is alerted, but is not a threat. Additionally, if the sensitivity that is optimal for the terminal environment is used in the enroute environment, traffic alerts for intruders may be issued too late to prevent a collision or may require extreme maneuvering.
In previous systems, four methods have been used to adjust traffic alerting system's sensitivity. The first is manual control, where the pilot manually sets the sensitivity level. The second is based on pressure altitude. On departure, the pressure altitude increase is used to change from a terminal sensitivity to an enroute sensitivity. On approach, the pilot must manually set the destination airport elevation and as the plane descends towards the airport elevation, the sensitivity changes from enroute to terminal modes. This second method does not work well if an aircraft descends enroute but not near the destination airport. The third method involves the selection of a landing-related aircraft system, such as flaps or landing gear. When the landing system is deployed, indicating the pilot's intention to land, the traffic system changes sensitivity. This method does not work on aircraft with fixed landing gear or where the position of the landing gear or the flaps cannot be determined by the traffic system. The fourth method uses radio altitude to filter traffic on the ground, but only once a plane has descended below a certain altitude (often 2500 feet).